In semiconductor fabrication, a plurality of layers are applied to or removed from wafers during the fabrication process. The layers can also be provided in special test regions within a plurality of identical recurring pattern elements. With increasing integration density, requirements in terms of the quality of the layers applied onto the wafers also rise. This means that during the production of a wafer with its plurality of process steps and plurality of layers to be applied (e.g. SiO2, SiNO3, polysilicon, etc., or the like), reliable detection of layer thicknesses must be possible. The existing art discloses a plurality of optical measurement arrangements, operating on the principle of spectrophotometry, for measuring layer thicknesses and associated material parameters. For this, a broadband light beam is focused almost perpendicularly onto the specimen, and the reflected light component is measured. In the spectrograph, that light component is in turn imaged via a grating in wavelength-selective fashion onto a CCD chip. Using a model that contains the optical parameters of the specimen as a function of wavelength, the parameters can be determined by way of a fit to the theoretical spectrum.
These measurement arrangements can be used in particular when thin layers and their optical parameters, for example the refractive index or extinction factor of single- or multilayer systems, need to be measured on wafers. In wafer manufacture specifically, the effort toward ever-thinner layers also means more stringent requirements in terms of the accuracy of the optical measurement arrangement with which the layers can be checked for exactness.
Also known from the existing art for performing such measurements are spectroellipsometers. With these, both the layer thickness and the optical parameters of transparent layers can be determined very accurately. This is done by directing a linearly polarized broadband light beam onto the specimen at an angle. The reflected beam is examined using an analyzer and a spectrograph to identify changes in polarization. The analyzer rotates, allowing only the particular light component that is vibrating in the corresponding polarization plane to strike the spectrograph. That light component is split in wavelength-selective fashion in a spectrograph using a grating, and imaged onto a CCD chip. The spectrum that is obtained then, by way of a model, allows filtration of the optical parameters and the layer thickness.
In order to obtain accurate measurement results, the above-described optical systems usually must first be calibrated. One possibility for calibrating a spectrophotometer is proposed in U.S. Pat. No. 5,771,094, in which a light source having a plurality of spectral lines is used. These spectral lines are imaged onto a CCD, and the lines are each allocated to specific pixels of the CCD. This results in an allocation of the pixel position on the CCD to the known wavelengths of the spectral lines, so that the relationship or calibration function between each pixel on the CCD and the associated wavelength value is determined. The calibration can be further verified by subsequent measurement of a known specimen.
In order to enhance measurement accuracy and, if applicable, to obtain additional information about the specimen, it is known from the existing art to use optical combination devices with which it is possible to carry out two or more examination methods. In this context, in particular, an ellipsometer and a spectrophotometer are advantageously implemented in one device. An optical measurement system for this purpose is known from U.S. Pat. No. 6,567,213. This combination measurement system allows results from the various measurement systems that are present to be combined, thus yielding more accurate results. A prerequisite for good results, however, is that an exact calibration of the measurement systems be performed before the specimens are measured. This is usually done by measuring a known substrate using all the available optical measurement methods, and calibrating each measurement method using the known specimen data.
Because an independent calibration of the two measurement devices leads to differences in measured values, exclusive use of the known calibration methods for such cases leads to unsatisfactory results.